1. Field of the Invention
The present invention relates to a video display device and a video display method capable of suppressing burn-in in areas bordering on high-luminance portions on a screen.
2. Description of the Prior Art
In recent years, the progress in semiconductor processing technologies has facilitated digital signal processing of video signals. Also, in recent time, in place of conventional cathode ray tubes, video display devices with fixed number of pixels such as LCD (Liquid Crystal Display) devices and plasma display devices have matched the Japanese small houses because of their slimness and have been widely spreading.
The video signaling system to be used for those video display device is required to display signals of various types of formats such as an HDTV (High Definition Television) broadcast system which is a next generation high-definition system, not to mention television standards like NTSC (National Television System Committee) signals.
In these various types of formats, the numbers of pixels to be handled are different. When displaying video signals of various types of formats with different numbers of pixels, an analog display device such as a conventional cathode ray tube etc. can easily cope with them if a deflection rate of the electronic beam is changed according to the number of pixels per one scanning time period.
However, since the number of pixels to be handled is fixed in the previously described display devices, a conventional analog technology as in the case of a cathode ray tube cannot be used. Therefore, in order to display signals of those various types of formats on a display device whose number of pixels to be handled is fixed, it is essential to optionally change the number of pixels by digital signal processing.
In order to change the number of pixels, it is commonly performed to produce interpolation pixels from the inputted pixel data by interpolation. As such interpolation methods, roughly three types of methods as follows are known.
1. Nearest Neighbor Interpolation
This is a method to pick up data nearest to the position of the pixels after the change of the number of pixels from the pixel data of the inputted pixels, and its hardware construction can be achieved by a very simple logic operation. However, the image quality after the change is considerably degraded. When scaling down, fine lines may disappear or small objects may be deformed. When scaling up, jaggies may appear in the peripheral area.
2. Bilinear Interpolation
This method is to pick up data at two points nearest to the position of the pixels after the change of the number of pixels from the pixel data of the inputted pixels and performs interpolation by using data at the two points. The picture quality is less degraded in this method than in the above nearest neighbor interpolation. However when scaling down to less than ½, a phenomenon called a pixel drop out occurs and the image quality is degraded. Further, since a loose low-pass filter is used in this method, the image is blurred as a whole, particularly in the edge portions. Also, the amount of operation required is greater than that of the nearest neighbor interpolation.
3. Cubic Convolution Interpolation
This method uses the value to which linear filtering is applied by utilizing also the value of the more distant pixel as a value of the input pixel. The amount of operation required in this method is far greater.
Of the three interpolation methods, the image quality after the scaling of the image is best in the cubic convolution interpolation, followed by the bilinear interpolation and nearest neighbor interpolation in that order. On the other hand, the least amount of operation is required in the nearest neighbor interpolation, followed by the bilinear interpolation and cubic convolution interpolation in that order. Considering the above, the bilinear interpolation method is adopted in many cases.
In the bilinear interpolation, however, images tend to be blurred. Therefore, for example, a method called sharpness processing is used to improve such blurring. It is an edge enhancement by using a method to find a secondary differential waveform and adequately add this waveform to an original waveform. By performing such sharpness processing, blurring is improved to some extent. However, it also brings about pre-shoots or overshoots.
On the other hand, Hi-Vision (HDTV) broadcasting, which is expected as next-generation TV broadcasting, is upgraded from experimental broadcasting to test broadcasting and its broadcasting hours are extended. Both the vertical and horizontal resolutions of the above Hi-Vision TV are twice as high as those of the current NTSC system television. The aspect ratio (length-to-width ratio) of its screen is a wide aspect of 16:9 and is more horizontally oriented than the aspect ratio 4:3 of the current NTSC system.
Therefore, when showing a picture of a conventional NTSC-system aspect ratio on a display device of a wide aspect ratio, it was necessary to trim upper and lower ends of the picture, or to widen the picture in the right and the left direction, or to provide a black blank zones containing no picture in the right and left ends. However, the first two methods could damage the original picture quality, particularly when viewing a movie, and could give discomfort to a viewer. Therefore, the method to provide blank zones is commonly used.
However, when providing such blank zones, since they are black zones, there often occurs a considerable difference in luminance between themselves and the picture portion. In addition, when the picture is processed by the above bilinear interpolation, sharpness processing is often applied to improve the blurring of the picture. In such a case, as in a graph of horizontal axis/luminance distribution shown in FIG. 3, overshoots 30a and 30b as well as undershoots 30c and 30d tend to take place in video signals. The luminance difference further increases in the picture portions bordering on the above blank zones. Therefore, display devices such as an LCD and a plasma display with the fixed numbers of pixels tend to have burn-in in picture portions bordering on the blank zones on a screen. In particular, the plasma display has such a tendency.
Conventionally, the following technologies are known to prevent burn-in taking place in a display device.
According to Japanese Patent Application Laid-open No. 08-248934 (Patent Document 1), burn-in of a display device is made inconspicuous by blurring the boundary portion between a lighting and a non-lighting while moving a display screen by the amount equivalent to a required number of pixels after the lapse of a required time.
According to Japanese Patent Application Laid-open No. 10-222125 (Patent Document 2), when showing a picture whose aspect ratio is 4:3 on a screen of a PDP display with a wide aspect ratio, a television video signal is displayed on the display part of the display screen and the outer portions (blank zones) in both ends of the display part are made to emit light with the averaged brightness of the display part.
According to Japanese Patent Application Laid-open No. 2003-174601 (Patent Document 3), when showing a picture with an aspect ratio different from that of a video display screen, the video display position of the display section is moved, which makes the slope of burn-in at the boundary between the video display parts and the mask parts gentle.
The method to move the video display position as in the patent documents 1 and 3 may give discomfort to a viewer. On the other hand, the method to raise the brightness of the blank zones as in the patent document 2 makes it difficult to visually distinguish the blank zones from the picture portion, which is not comfortable for a viewer.
Further, the burn-in is not limited to the one taking place in the areas bordering on the blank zones. It also takes place in other picture portions when a high-luminance state lasts at the same position for a long time. In such picture portions, it is hard to adopt previously described methods of patent documents 1 to 3 from the aspect of picture quality.